tests: validate "patch" and "ed" commands once, print meaningful messages

[marc-hb]

macOS' /usr/bin/patch and GNU patch have very subtle incompatibilities that cause only some "more advanced" tests to fail in obscure and very time-consuming ways - while other tests pass. In some cases (depending on test threads racing), the lack of newlines in some test data even causes the whole test suite to stall.

This fix runs patch -version (only once), makes sure the output starts with "GNU patch" and shows a meaningful assert message when not. It also looks for gpatch instead of patch on macOS and shows a meaningful assert message if either is missing.

Fixes: #225

This also provides faster and better feedback when ed is missing (see #39) and implements a portable and basic check.

Last but not least, this new code is generic enough to support the validation of any other test dependency in the future.

[marc-hb]

This seems unrelated:

https://results.pre-commit.ci/run/github/341787720/1778103980.1j7-ePVIRuecaKMwwUj_Qg

.pre-commit-config.yaml is not a file

[marc-hb]

With this fix, the /opt/homebrew/opt/gpatch/libexec/gnubin PATH change that can be seen in 44c195c is not needed anymore and can be dropped. Installing gpatch is still required of course.

[marc-hb]

Except for this mysterious .pre-commit-config.yaml is not a file, all tests passed in https://github.com/uutils/diffutils/actions/runs/25462948803 for initial commit 877869e

I just force-pushed practically identical commit 05c139b as a attempt to re-run whatever failed with .pre-commit-config.yaml is not a file. That didn't work: it did not re-run anything. https://results.pre-commit.ci/run/github/341787720/1778165671.6uM0eN6KQY6YzmMZAfkOUQ is still there.

Also, I lost the successful/green status of https://github.com/uutils/diffutils/actions/runs/25462948803, now workflows need to be approved again :-(

EDIT: thanks for re-approving!